1. Field of the Invention
The present invention relates to a method for forming an image for developing an electrostatic latent image in an electrophotographic method or an electrostatic recording method.
2. Description of the Related Art
In the image forming method utilizing electrophotography, generally, an image is formed by using an electrostatic latent image carrying member in a drum or belt form (hereinafter, sometimes referred to as an electrophotographic photoreceptor) in the following manner. After uniformly charging the surface of the electrostatic latent image carrying member with a charging unit, the surface of the electrostatic latent image carrying member is irradiated in the form of an optical image corresponding to image information to form an electrostatic latent image, and then the latent image is developed with a developing unit for feeding a developer according to a developing method to form a toner image. Thereafter, the toner image is electrostatically transferred to a recording sheet, such as paper, or electrostatically transferred to a recording sheet through an intermediate transfer material. Unnecessary matters, such as the non-transferred toner, remaining on the surface of the electrostatic latent image carrying member after transfer are then removed by cleaning according to a cleaning method, and the electrostatic latent image carrying member is prepared for the next image formation process.
In the cleaning method of the aforementioned image forming method, such a method is frequently employed that a cleaning blade formed, for example, with rubber, is pressed on the surface of the rotating electrostatic latent image carrying member to remove and recover the unnecessary matters, such as the non-transferred toner, and thus the following problems arise. The cleaning blade gradually abrades the surface of the electrostatic latent image carrying member to deteriorate the characteristics of the electrostatic latent image carrying member, and thus the service life thereof is reduced, which is a disadvantageous factor for improvement in service life. Furthermore, in general, the toner removed and recovered with the cleaning blade is stored in a recovery container disposed inside the apparatus and then discarded periodically, or is stored in a recovery part attached to a disposable cartridge member and then discarded along with the cartridge having the recovery part. The discard of the toner is not preferred from the standpoint of environmental protection and is necessarily refrained.
In recent years, accordingly, an image forming method using a so-called blade cleaner-less system equipped with no blade cleaning mechanism has been proposed for avoiding abrasion of the surface of the electrostatic latent image carrying member with the cleaning blade. In the case where the cleaner-less system is employed, a so-called spherical toner, i.e., a toner formed into a spherical form, for example, by an emulsion polymerization and integration process or a suspension polymerization process. According to the process, the transferring rate of the developed image on the electrostatic latent image carrying member is improved to reduce the amount of the non-transferred toner remaining on the surface of the electrostatic latent image carrying member, whereby the step of cleaning the surface of the electrostatic latent image carrying member after transferring can be omitted.
Such a measure is also employed that the non-transferred toner remaining on the surface of the electrostatic latent image carrying member is recovered through a developing roll for the developing method (i.e., developing device cleaning) to make the device function as a cleaning device.
However, the cleaner-less image forming method involves the following problems. The surface of the electrostatic latent image carrying member suffers attachment of discharge products, such as a nitric acid compound, formed upon discharge in the charging step and the transferring step, whereby the resistance of the surface of the photoreceptor is decreased, and the electrostatic characteristics thereof are deteriorated, so as to cause, for example, white dropout (image blur). There is such a tendency that the white dropout due to attachment of the discharge products conspicuously appears under high temperature and high humidity conditions.
As a cleaner-less image forming method that avoids the attachment of the discharge products and the contamination of the surface of the electrostatic latent image carrying member, for example, JP-A-10-240004 discloses such a method using the developing device cleaning that the surface of the electrostatic latent image carrying member is abraded by changing the peripheral velocity of the developing roll between the step of forming an image and the step of cleaning. JP-A-10-254291 discloses such a method that a unit for scraping the surface of the electrostatic latent image carrying member (such as a brush, a roller or a web in contact with the surface of the electrostatic latent image carrying member) is provided between the transferring unit and the charging unit, whereby the surface of the electrostatic latent image carrying member is abraded in a prescribed amount. However, these techniques are associated with abrasion of the electrostatic latent image carrying member to reduce the service life of the electrostatic latent image carrying member, and thus is disadvantageous in improvement of the service life of the electrostatic latent image carrying member.
It is the current status that the charging technique is shifted from the conventional non-contact charging method utilizing corona discharge to the contact charging method using a member in contact with the electrostatic latent image carrying member.
In the contact charging method, an electroconductive elastic roller is made in contact with the electrostatic latent image carrying member, and the electrostatic latent image carrying member is uniformly charged by applying a voltage to the electroconductive elastic roller. In the contact charging method, discharge is liable to occur immediately before and immediately after the contact of the charging device with the electrostatic latent image carrying member. Therefore, discharge products, such as a nitric acid compound, are liable to form to cause image white dropout (image blur) particularly under high temperature and high humidity conditions, as compared to the conventional corona discharge method.
Furthermore, since the charging device and the electrostatic latent image carrying member are in contact with each other, a toner is fixed and accumulated on the charging device, the transferring member and the electrostatic latent image carrying member in the case where the toner remains even in a slight amount due to insufficient transfer and fogging upon development, whereby image defects occur due to charging of transferring failure in long term use.
Consequently, in the image forming method using no elastic blade or using no cleaning mechanism, particularly in the image forming method using a contact charging device, such a function is necessary that effectively removes the discharge products and the toner remaining in a slight amount.
It has been also proposed that, instead of the elastic blade, a brush is pressed on the electrostatic latent image carrying member with a small pressure to clean the electrostatic latent image carrying member. The cleaning method using a brush is advantageous in such a point that the surface of the electrostatic latent image carrying member is suppressed from being deteriorated, and it is sufficient to collect the toner remaining in a slight amount while the toner collecting amount is smaller than that of the elastic blade. However, there is such a problem that collecting power of the fixed remaining toner is small in comparison to the elastic blade.
There have been proposed that in an image forming method using a cleaning unit, the surface of the electrostatic latent image carrying member is coated with a cleaning assistant for improving the cleaning property or a lubricant for reducing flaws and abrasion of the surface of the electrostatic latent image carrying member on the cleaning part. However, these measures cannot be an effective solution due to the following problems.
For example, JP-A-60-225870 discloses an image forming method, in which a cleaning assistant coating unit for coating the aforementioned cleaning assistant with a foamed body having the cleaning assistant attached thereto is disposed at a position on a periphery of an electrostatic latent image carrying member on a downstream side of the blade cleaning system and an upstream side of the charging unit. However, in the case where the contact charging system, in which a charging member charges the surface of the electrostatic latent image carrying member by making it in contact therewith, is employed in this image forming method, there is such a possibility that the cleaning assistant is attached to the surface of the charging member to induce charging failure, which brings about deterioration in image quality.
JP-A-10-142897 discloses an image forming method, in which a coating mechanism for coating the aforementioned lubricant on a charging roller is provided, and a lubricant coating assisting member, such as a blade, in contact with the surface of the electrostatic latent image carrying member is provided on a periphery of the electrostatic latent image carrying member on a downstream side of the charging roller and an upstream side of a developing unit, whereby the lubricant fed onto the electrostatic latent image carrying member is coated into a film form before reaching the developing unit. In the image forming method, however, since the lubricant coating assisting unit is provided to be made in contact with the surface of the electrostatic latent image carrying member after the charging unit and after coating the lubricant, there is such a possibility that uniformity of the surface potential of the electrostatic latent image carrying member is deteriorated due to frictional charge between the assisting member and the surface of the electrostatic latent image carrying member, so as to cause deterioration in image quality.